Temperature control of a chemical reaction is often the key to obtaining desired products. Where the temperature is controlled, generally the reaction kinetics are controlled. Where the reaction kinetics are controlled, undesired intermediates and byproducts can be diminished or avoided. Traditional temperature control of industrial reactors is generally attained in one of two ways. One method is to control the temperature of the reactants as they enter the reactor. This method fails to address the heat of reaction, which is often responsible for the majority of heat produced or absorbed in a reaction. The heat of reaction can then alter the temperature of the reactants to produce undesirable products. This is especially true for tank reactors.
Conversely, endothermic reactions require the addition of heat during the reaction to maintain the temperature of the reactants. Again, pre-adjustment of the temperature of reactants fails to adequately address this situation. Further, complicated production processes may have exothermic and endothermic reactions taking place (usually at different times) as reactants are added or products withdrawn. Pre-adjustment of reactant temperature is clearly totally inadequate in such situations.
A second method of temperature control of industrial reactors involves the placement of a jacket around the outside of the reaction vessel. In such a case, a fluid of desired temperature is passed through the jacket, thereby cooling or heating the reaction medium. The effectiveness of the jacket is limited by heat transfer properties which are in turn limited by mechanical design characteristics and geometry, including specifically vessel diameter and length. Material of construction, wall thickness, vessel diameter and length are critical design parameters for both strength and heat transfer. Unfortunately, however, heat transfer and mechanical strength are competing values in reactor design. For a given vessel diameter and length, the reactor wall may be thick enough to meet pressure and strength requirements, but too thick for optimal heat transfer between the jacket fluid and the reaction medium, as heat transfer is decreased with increased wall thickness. Where the reactor wall is thinned to improve heat transfer, the structural integrity of the vessel is diminished. This trade-off has historically been the source of design efforts seeking to gain maximum heat transfer efficiency while meeting mechanical strength requirements. If there is an increase in vessel diameter for a given length, the wall becomes weaker under internal pressure and weaker (to a higher order) under external pressure. Increasing vessel diameter, for a given length, also decreases (heat-transfer) surface to reacting medium volume, further inhibiting heat transfer mechanisms.
In the design of reactor cooling systems, two additional concerns arise when low temperatures are needed and cryogenic fluids are being contemplated for use as refrigerants. First, the temperature of the jacket fluid is calculated based on heat transfer requirements for a given reaction medium and reactor design. The required jacket fluid temperature is often below the freezing point of the reactor medium. As a consequence, the reactor contents can freeze along the inside of the reactor wall. The formation of “ice” results in a thicker wall overall and decreased heat transfer efficiency, as well as potentially inconsistent reactor medium composition, and in some cases, destruction of some reactants or products through freezing. Second, when a cryogenic fluid changes phase the vapor generated could occupy as much as 100 times the same volume as the liquid from which it originated. This large increase in specific volume can lead to erratic heat transfer mechanisms and, consequently, poor reactor medium temperature control.
Thus, there is a need for an improved apparatus for controlling the temperature of a reactor during operation that would allow for a (1) thin wall and resultant increased heat transfer to the contents and (2) increase of reactor size (diameter and length) without sacrificing the required mechanical properties of the reactor. Additionally, such an apparatus which prevents the build-up of frozen reactor contents would maintain high heat transfer efficiency and constant reactor medium temperature gradients, resulting in homogeneous and uniform reaction kinetics. The desired reactor would maximize the desired properties of high mechanical strength and high heat transfer efficiency, two qualities which have historically competed, regardless of size (i.e. diameter and length).